Users of wireless telephone services expect that their messages shall be reliably conveyed from the subscriber unit (SU)to the fixed equipment and from the fixed unit through the public switched telephone network (PSTN) to the intended receiver of the message. Digital radio transmission techniques have evolved for carrying the user's messages over the unreliable radio link with good reliability by introducing error correction and acknowledgement of received data. Uncorrectable or unacknowledged data may be retransmitted to ensure receipt of the message.
Some devices which use wireless telephone services require bit synchronization to maintain operation. One such device is a secure telephone instrument commonly called a STU III secure telephone (an example of which is the SECTEL 1500 model, available from Motorola, Inc., Government Electronics Group P.O. Box 9040 Scottsdale, Ariz. 85252). The STU III "black digital" protocol is defined in National Security Agency (NSA) document FSVS-211 for STU III instruments. Further, a protocol which is often employed in connection oriented protocols typically used on connectionless networks for data packet delivery is known as Transmission Control Protocol (TCP) and is particularly useful over complex communication networks. The standard for data formats and procedures in a TCP is specified in part of the TCP/IP suite of communications protocols and is defined in RFC 793, RFC 761, RFC 675 and other RFC's (Request For Comments) available from the Network Information Center at SRI International.
Briefly, TCP conveys data packets organized into a plurality of 8-bit octets (bytes) for each packet. The packets are numbered so that packets are delivered to the receiver in order even though some packets may have become disordered due to network delays or due to the need to request retransmission of a packet. Typically, such retransmission is based upon a lack of a positive acknowledgement by the receiver. See, for example CCITT Recommendation X-141 "General Principles for the Detection and Correction of Errors in Public Data Networks" (1984) for a discussion of the acknowledgement process (ARQ).
While the foregoing protocols provide reliable packet delivery, timely bit-synchronous data transmission is not realized. Bit-synchronous communication devices, such as the STU III variety, will lose synchronism without a reliable input of data bits.